1. Field of the Invention
The invention generally relates to a transmitter/receiver, and particularly to a transmitter/receiver for generating a transmitting data signal in synchronization with a received data signal. The invention has particular applicability to a transmitter/receiver for the Integrated Service Digital Network (ISDN).
2. Description of the Background Art
The Integrated Service Digital Network (referred to as ISDN hereinafter) is known as a system for implementing various kinds of communication such as telephone, facsimile communication, data communication and video communication by digitized networks. In conventional communication systems prior to ISDN, interfaces between user's terminal equipments and each of the communication networks have been adapted for fixed usages, such as use for telephone and use for data communication. However, in ISDN, a unified interface is defined for the purpose of the above mentioned various services. This interface is called the multipurpose user network interface and clearly defined by the International Telegraph and Telephone Consulative Committee (referred to as CCITT hereinafter).
An application of ISDN is shown in FIG. 7. Referring to FIG. 7, in ISDN, an ISDN exchanger 18 at a telephone office and ISDN terminal equipments at a user's home (TE) are connected through a telephone line 28. With a network termination equipment (NT) 100 provided at the user's home, the telephone line 28 and a 4-wire home bus are connected therewith. Each ISDN terminal is connected to the network termination equipment 100 through the 4-wire home bus 19. At the network termination equipment 100 and each input/output section of ISDN terminals, a transmitter/receiver 27 is provided as an interface circuit to transmit and receive data signals between each other through the 4-wire home bus 19.
As an example of an ISDN terminal, a simplified block diagram of a telephone 20 is shown in FIG. 8. Referring to FIG. 8, the telephone 20 comprises the transmitter/receiver 27 connected to the 4-wire home bus 19 through a signal transmit/receive transformer 1, an LAPD (Link Access Procedure D channel) controller 22 for performing the layer 2 function of an ISDN basic interface, a microprocessor 23 for performing the layer 3 function of the ISDN basic interface, a key pad 24, a CODEC (coder/decoder) device 25 for coding and decoding audio signals, and a handset 26. The transmitter/receiver 27 is provided to implement the layer 1 function of the ISDN basic interface. The 4-wire home bus line 19 comprises a transmitting bus 19a for transmitting transmitted signals from the telephone 20 and a receiving bus 19b for transmitting a received signal to be applied to the telephone 20.
In operation, a received signal transmitted through the receiving bus 19b is received by the transmitter/receiver 27 through the transformer 1. The received signal is applied to the CODEC device 25 through the LAPD controller 22. The CODEC device 25 converts the received signal into a sound signal and then applies the converted signal to the handset 26. The sound signal generated from the handset 26 is converted to a digital signal by the CODEC device 25. The converted digital sound signal is applied to the transmitter/receiver 27 through the LAPD controller 22. The transmitter/receiver 27 supplies the converted digital sound signal to the transmitting bus 19a as a transmitting signal through the transformer 1.
The conventional transmitter/receiver 27 in FIG. 8 is shown in FIG. 9. Referring to FIG. 9, this transmitter/receiver 27 comprises a receiver circuit 2 connected to the receiving bus 19b through a receiving transformer 1a, a data buffer 3 for holding a received data signal S3, a phase locked loop (referred to as PLL hereinafter) circuit 4 for generating a periodical timing signal S5 in synchronization with the received data signal S3, a data buffer 6 for holding transmitting data Dt applied from the LAPD controller 22 in response to the periodical timing signal S5, and a driver circuit 7 for driving the transmitting bus 19a in response to the held transmitting data Dt.
In operation, a received data signal is applied to the receiver 2 through the receiving bus 19b and receiving transformer 1a. The receiver 2 waveform-shapes the received data signal and applies the waveform-shaped signal S3 to the data buffer 3 and the PLL circuit 4. The data buffer 3, after holding the applied data signal S3, transmits the held signal to the LAPD controller 22 as received data Dr. The PLL circuit 4, in response to the received data signal S3, generates a periodical timing signal S5, and applies it to the data buffer 6. The data buffer 6, in response to the applied timing signal S5, holds the received data Dt from the LAPD controller 22. The driver circuit 7, in response to the transmitting data held by the data buffer 6, drives the transmitting bus 19a through the transmitting transformer 1b.
The change in timing of signals and the waveforms in the circuit of FIG. 9 are shown in FIG. 10. Referring to FIG. 10, the received data signal S1 transmitted through the receiving bus 19b, as indicated in FIG. 10, has had its waveform changed. This change of waveform depends on the frequency characteristic of the receiving bus 19b. The receiver 2, upon receiving this received data signal S1, outputs the waveform shaped signal S3. Therefore, as indicated in FIG. 10, the signal S3 has delay .DELTA.t3 in comparison with the signal S1. Moreover, the PLL circuit 4, in response to this signal S3, generates the periodical timing signal S5 and the data buffer 6, in response to the signal S5, holds the transmitting data Dt. Since the driver circuit 7 drives the transmitting bus 19a in response to the transmitting data held within the data buffer 6, the transmitting signal S2 which appears on the transmitting bus 19a is further delayed by time .DELTA.t4 compared to the rising timing of the signal S5. As a result, the transmitting signal S2 on the transmitting bus 19a is to be delayed by time .DELTA.t5 (=.DELTA.t3+.DELTA.t4) compared to the rising timing of the received signal S1. The time delay .DELTA.t4 is caused mainly by the driver circuit 7. This is because the driver circuit 7 has to drive the transmitting bus 19a having a large load.
Referring to FIG. 11, the connection between the network termination equipment (NT) 100 and the ISDN terminals TE1 and TE2 is shown. The terminals TE1 and TE2 are connected to the network termination equipment 100 through the transmitting bus 19a and the receiving bus 19b respectively. Each of the terminals TE1 and TE2 and the network termination equipment 100 is provided with the transmitter/receiver 27 for transmission and reception of data signals.
The two terminals TE1 and TE2 connected to the network termination equipment 100 can be used simultaneously. That is, data are transmitted and received by the two terminals TE1 and TE2 through time division. Since the transmission and reception of data are performed through time division, the ISDN terminal is generally needed to generate a transmitting data signal in synchronization with a received data signal. This requirement is defined in the above mentioned recommendation I.430 (basic user network interface) by CCITT.
FIG. 12 shows a relation between generation timings of a received data signal RX1 and a transmitting data signal TX1 at one ISDN terminal. Generally, an ISDN terminal generates the transmitting data signal TX1 in response to a periodical timing signal generated based on the received data signal RX1. Therefore, as indicated in FIG. 12, the change timing of the transmitting data signal TX1 is delayed by .DELTA.t1 from the change timing of the received data signal RX1. According to the recommendation I.430 by CCITT, this time delay .DELTA.t1 is required to be within the range between -7% and +15% of one bit period. This time delay .DELTA.t1 is referred to as a total phase deviation in the recommendation I.430.
Generally, more than 2 ISDN terminals are connected to one network termination equipment through a 4-wire home bus. Two of the ISDN terminals can be used simultaneously. That is, these two terminals transmit and receive data signals through one network termination equipment through time division. For example, as indicated in FIG. 11 the case in which the two terminals TE1 and TE2 are used simultaneously, is described in the following.
The terminal TE1 outputs the transmitting data signal TX1 in synchronization with the received data signal RX1. Likewise, the terminal TE2 outputs the transmitting data signal TX2 in synchronization with the received data signal RX2. The received data signals TX1 and TX2 have the above mentioned delay compared to the received data signals RX1 and RX2. Two transmitting data signals TX1 and TX2 are overlapped on one transmitting bus 19a, through which the signals are transmitted to the network termination equipment 100. Therefore, as shown in FIG. 13, when the transmitting data signals TX1 and TX2 having the phase difference .DELTA.t6 are overlapped on the transmitting bus 19a, a transmitting data signal TX (=TX1+TX2) having the waveform as shown is obtained. This transmitting data signal TX indicates a stable value during the time .DELTA.t7 in the central part of each pulse waveform.
As can be understood from the waveform of the transmitting data signal TX shown in FIG. 13, it is noted that the larger the phase difference .DELTA.t6 between two transmitting data signals TX1 and TX2 becomes, the shorter the period .DELTA.t7 representing the stable value in the overlapped signal TX becomes. In other words, the above mentioned recommendation I.430 by CCITT requests that the phase difference .DELTA.t1 between the received data signal RX1 and the transmitting data signal TX1 at each terminal lies within the given range, in order to secure the length of this stable period .DELTA.t7.
As shown in FIG. 10, the conventional transmitter/receiver 27 outputs the transmitted data signal S2 delayed by the phase difference .DELTA.t5 compared to the received data signal S1. As this phase difference .DELTA.t5 becomes larger, the transmitter/receiver satisfying the requirement of the recommendation I.430 can not be obtained. As a result, the period .DELTA.t7 shown in FIG. 13 becomes shorter and a transmitting data signal generated from each terminal will not be transmitted to the network termination equipment 100 correctly.